1. Field of the Invention
This invention relates generally to pressure control systems and more particularly to vacuum pressure control systems for chemical vapor deposition apparatus.
2. Description of the Prior Art
Chemical vapor deposition (CVD) is a process for depositing a solid material from a gaseous phase onto a substrate by means of a chemical reaction. The deposition reaction involved is generally thermal decomposition, chemical oxidation, or chemical reduction. In thermal decomposition, for example, organometallic compounds are transported to the surface of the substrate as a vapor and are reduced thereat to the elemental metal state on the substrate surface. Although such deposition on substrates can be utilized for a variety of purposes, CVD apparatus is particularly useful in manufacturing solid state electronic devices and energy conversion devices.
For reference, chemical vapor deposition of electronic materials is described, for example, in the following publications: T. L. Chu, et al., J. Bac. Sci. Technol., 10, 1 (1973), and B. E. Watts, Thin Solid Films, 18, 1 (1973). These publications describe the formation and doping of epitaxial films comprised of such materials as silicon, germanium and gallium arsenide. In the field of energy conversion, the CVD process provides materials for nuclear fission product retention, solar energy collection and super conduction. A summary of the chemical vapor deposition field is furthermore provided by W. A. Briant, in "The Fundamentals of Chemical Vapor Deposition", Journal of Materials Science, 12, 1285, (1977).
The deposition parameters of temperature, pressure, the ratio of reactant gases and amount of distribution of gas flow critically determine the deposition rates and the ability of a particular system to provide the desired uniformity and quality of deposition. The limitations of prior art systems stem from their inability to adequately control one or more of these factors or from deposit's contamination.
One known specific type of improved CVD apparatus comprises an invention entitled, "Chemical Vapor Deposition Apparatus", U.S. Ser. No. 528,193, Bryant A Campbell, et al., filed on Aug. 31 1983, and which is assigned to the assignee of the present invention. In that apparatus, an inner deposition chamber having gas distribution means for introducing gas into the inner chamber and removing gas therefrom and a vacuum chamber means surrounding the inner deposition reaction chamber and spaced from the walls thereof for maintaining a minimum vacuum therein is described. The vacuum chamber means further comprises a domed housing and a base cooperating therewith and wherein the material of the domed housing and the base are substantially transparent to radiation. Radiant heating means are positioned over the outer surface of the domed housing and the base surrounding the inner deposition chamber for providing precisely controlled temperatures in the reaction chamber. The radiant heating means and the outer surface of the domed housing and base are in a non-conducting relationship. The radiant heating means preferably have the same temperature achieved by having the same cross sectional areas and currents. The domed housing has a base which engages a support plate with seals being positioned between the base and the support plate to form a vacuum seal. Cooling means engage the outer wall of the domed housing between the base and the portion thereof surrounding the deposition chamber for removing heat therefrom to protect the seals. The quartz vacuum chamber base furthermore has an outer domed portion and an axially concentric inner cylindrical portion integral therewith. Such a structure provides a chemical vapor deposition system which provides a more uniform temperature in the inner deposition reaction chamber.
With respect to conventional CVD apparatus as well as the system referred to above, various servo types of control systems have been suggested for controlling the extremely low pressures (0-10 torr) which are required in such apparatus. Typically, the internal pressure is monitored by a transducer which operates to generate an electrical analog signal indicative of the internal pressure. This signal is compared with another electrical signal indicative of a desired pressure or set point using a proportional integral derivative type of controller. An analog error signal is then generated which is utilized to control the power applied to an electrical motor coupled to a vacuum pump whereupon the vacuum pump's speed and vacuum pressure are changed until there is substantially zero error between the actual pressure and the desired or set point pressure.
With this type of pressure control system, a serious limitation exists due to the fact that the analog signal is sensitive to environmental noise and particularly if the vacuum pump connected to the vacuum chamber is remote from the CVD apparatus. At sensitive levels of operation, it has been found impossible to prevent the fluctuations, even though shielding of interconnecting electric cables have been utilized. This factor presents a limit to the levels of coating uniformity which can be realized and accordingly the resulting yield.
Other known means for controlling pressure in these types of vapor deposition systems are disclosed in: IBM Technical Disclosure Bulletin, 1975, at page 2082 entitled, "Controlling (Reaction) Pressure In CVD Tools", by R. E. Chappelo, et al., and Japanese Pat. No. 56-152,738, dated Nov. 26, 1981, and being assigned to Mitshbishi Electric Company. In the former publication, a pressure transducer provides an output signal to a differential amplifier which compares the measured signal against a signal representing the desired pressure. The output of the differential amplifier is used to control an element which is used to modulate the position of an electrically controlled valve. In the latter patent, a branch pipe is connected in parallel to the reactor tube between the gas feeder and the evacuator and a sensor is inserted into the branch tube to measure the pressure in the tube. Depending upon the measured pressure, i.e. the vacuum in the tube, a rotary pump of the evacuator is controlled to obtain the desired pressure in the tube.
Additionally, it should also be pointed out that pulsewidth modulation servo control systems are generally known. Typical examples include: U.S. Pat. No. 3,686,557, entitled, "Floating Pipe Electronic Servo Motor Process Controller", issued to John Futamora, on Aug. 22, 1972; U.S. Pat. No. 3,874,407, entitled, "Pulse Width Modulation Control For Valve", which issued to R. F. Griswold on Apr. 1, 1975; U.S. Pat. No. 4,146,828, entitled, "Pulse Modulated Servo Amplifier", which issued to R. E. Ross, et al. on Mar. 27, 1979; U.S. Pat. No. 4,204,143, entitled, "Pulse Width Modulated Power Amplifier For Direct Current Motor Control", which issued to R. D. Coleman on May 20, 1980; and U.S. Pat. No. 4,236,106, entitled, "Automatic Pulse Controlled Servo Control System", which issued to D. M. Davis, et al. on Nov. 25, 1980.
Other servo type control systems of which applicants are also aware include: U.S. Pat. No. 3,445,739, entitled, "Variable Speed Control For Position Regulator Motor", which issued to L. F. Martz on May 20, 1969; U.S. Pat. No. 3,962,620, entitled, "Switching Apparatus", which issued to W. E. Dion on June 8, 1976; U.S. Pat. No. 3,979,654, entitled, "Process Control System Using A Two Wire Remote Control System", which issued to E. H. Guicheteau, et al. on Sept. 7, 1976; and U.S. Pat. No. 4,473,783, entitled, "Power Control Circuit", which issued to G. S. Morez on Sept. 25, 1984.
In the parent copending application entitled, "Pressure Control System For Chemical Vapor Deposition Apparatus", Ser. No. 813,915, referenced above, there is disclosed a pulsewidth modulated control of a 30 silicon controlled rectifier bridge type AC to DC converter which supplies power to a DC motor controlling the vacuum pump which operates to remove gases continuously fed into chemical vapor deposition apparatus during its operation. The actual pressure within the vapor deposition apparatus is sensed by a manometer type transducer which generates an electrical signal corresponding to pressure. This actual pressure signal is fed to an error signal amplifier along with an electrical signal corresponding to the desired pressure set point. An absolute voltage is provided at the output of the error signal amplifier which increases or decreases from the desired set point value depending upon the actual pressure measured. This signal is then commonly coupled to three phase comparators (0.sub.1, 0.sub.2 and 0.sub.3) which additionally receive respective ramp signal generated from the 30 AC power signal fed to the converter. The respective outputs of the three comparators comprise three phases of pulsewidth modulated pulses which are fed as control signals to the SCR bridge converter for controlling the speed of the DC motor coupled to the vacuum pump.